We propose to study the developmental parameters which guide the early growth and determination of the head of Drosophila melanogaster. Clonal analysis will be accomplished through use of genetic cell markers and X-ray induced mitotic recombination. The developmental restrictions imposed on head anlage cells are detected by use of clones whose cells have an autonomously higher division rate than that in neighboring cells. These restrictions lead to a developmental compartmentalization of contiguous cells beginning shortly after a blastoderm and continuing in a sequential manner for several cell divisions. A detailed fate map of the blastoderm head cells has aided in the establishment of these compartments. Further analysis of head development is being undertaken using the extra eye mutant a mutation that can evoke the production of an extra half-head on the vertex of the fly, and the opthalmoptera mutation, a mutation that can cause the formation of the anterior part of the wing in tissue that normally becomes compound eye. The specificity of neuronal connections is being investigated in the extra eye mutant as well as homeotic mutations affecting the legs of Drosophila.